28 February 2009

The winner has just been announced for RIBA's River Soar Footbridge contest. The new bridge, a short span over the river in Leicester, is part of a regeneration project and connects the National Space Centre with proposed housing at Wolsey Island.

The design is a partially underslung suspension bridge, which I'd guess for many people will be reminiscent of the London Millennium Bridge, although it doesn't go for such an expensively shallow cable catenary. There are also similarities to the Arup / Wilkinson Eyre designed "living bridge" in Limerick although that one is completely underslung. The Arup design also has the advantage that it was (so far as I can tell) designed as a self-anchored bridge, removing the expense of anchorage foundations.

Happold's design is also more expensive than a conventional suspension bridge due to the need to brace the main cables apart rather than just hang the deck below them, but it certainly results in a very elegant design.

Unlike the far larger Metro West bridge, it's an appropriate structural design for the context. The support towers will require oversized foundations (unlike a conventional suspension bridge, there are no ground anchorages, with the main cables held by the towers acting as cantilevers), but that won't be apparent visually.

25 February 2009

In April last year, the Irish Rail Procurement Agency (RPA) launched a competition with the Royal Institute of Architects of Ireland (RIAI) to find a design for a new bridge to carry the proposed Metro West light rail line over the Liffey Valley. Like other bridge contests run by an architects' trade body, it was not without its flaws (engineers couldn't enter without an architect in tow, for example, however experienced in bridge design), but generally it was one of the better planned contests of recent years, with half-decent rewards for the entrants and a prequalification hurdle to keep out the students and chancers.

The location was sensitive environmentally, with a strong desire to avoid major construction impact on the river valley floor. Certain parts of the floor were specifically marked "out of bounds" on the project plan. The client also aspired to a signature bridge which would give pedestrians new views of the valley, while making as minimal visual impact as possible.

Around twenty-five firms made entries to the competition's first stage (if I find any online, I'll put a post together, but haven't spotted any yet). Five were shortlisted to the second stage, with their identities kept secret until a winner was declared.

The shortlisted entrants were:

Alan Baxter / Aedas

Arup / Heneghan Peng Architects

Buro Happold / Explorations Architecture

Flint and Neill / Foster and Partners

Scott Wilson Scotland Ltd. / B+M Architecture

The winner has just been announced as Happold / Explorations, with the semi-underslung suspension bridge design pictured below:

It's easy to see why this entry won, especially when you read the jury report[PDF].

None of the other shortlisted entries have yet been made public (if any of my readers were involved, feel free to email details to happypontist at gmail dot com!) However, the jury report implies that the other four entries all had supports within the Liffey Valley itself, whereas the Happold design spans right across (I seem to recall it's about 300m or so). You might think the other four took a reasonable approach, given that the competition brief explicitly permitted supports in the valley, and given that this was an obvious way to keep the spans within sensible economic boundaries and hence within the stated project budget.

However, it's no surprise that a design which avoids piers entirely would be looked on favourably.

One problem is that a suspension bridge does not necessarily keep construction activity out of the valley floor - on the contrary, in the way one is normally built, sections of deck are lifted vertically to be supported on the main cables i.e. in this case, from throughout the valley.

In contrast, bridges with valley supports can be built by the balanced cantilever method, limiting impact to only the base of the piers themselves. So as the winning bridge design progresses it will be interesting to see exactly how it gets built.

One entry appears to have been disadvantaged by being too tall and dominant (Flint and Neill). Without seeing the design it's hard to comment but the entry I was involved with in this competition ruled out any structure that projected significantly above the valley crest, so I sympathise with the jurors here.

Another was seen as too complex geometrically (Arup) which might lead to cost concerns, or aesthetic problems if it had to be simplified later.

But a suspension bridge, particularly one which is part underslung and hence requires substantial additional stiffening to prevent instability, is hardly the least cost solution - for this span it may in fact be the exact opposite.

Suspension bridges require substantial ground anchorages to restrain the main cables - the cost of which can be enormous if the ground is unsuitable. In its infancy, this type of bridge was shunned for use on railways following notorious failures and problems making them sufficiently stiff under high locomotive loads. This is not a problem that has gone away today, particularly for light rail bridges where the design is often governed by stiffness rather than strength.

So while it's a very nice design visually, if I were a juror I'd have been looking for the designer to convince me it had a real chance of being built within budget, without having to alter it visually (e.g. having to make the deck stiffer and hence thicker).

With that in mind, it may be worth noting that nobody on the jury (largely made up of RPA staff) appears to have been a specialist landmark bridge engineer, let alone a specialist in cable-supported bridges. The only outside engineering advisor was John Powers of Engineers Ireland, an electrical engineer. A failure to draw on that kind of specialist advice has been a glaring error in several previous bridge design competitions.

I would certainly hope that the design proves successful - it's striking and highly ambitious, and would become one of Ireland's greatest bridges if built. I'd also be the first to admit that it's somewhat ridiculous to try and dissect the design on the basis of a few website photos without access to the full design submission. But either way, it will definitely be interesting to see how it progresses in future.

Update 26 Feb: I gather the RIAI intends to publish all the shortlisted entries, and possibly some of the stage one entries as well, on their website. I'll post again when they become available.

Further update 26 Feb: I've replaced the images with higher-resolution versions and added one or two more. All images provided by the RIAI, with credit to Buro Happold / Explorations Architecture.

24 February 2009

There have been several schemes to build new bridges at Leamouth in east London (where the River Lea joins the River Thames) over the years. One such scheme, held in 2003, and promoted by Leaside Regeneration, involved a design competition for an opening footbridge. This was to provide a link into the somewhat isolated and downtrodden Leamouth peninsula (shown on the right) and encourage new development.

That design would have been spectacular and unusual if built - but also incredibly inefficient. In the closed position, it greatly resembles Santiago Calatrava's Puente del Alamillo, with a pylon which will carry substantial bending moments. As it opens, its weight is unbalanced at every stage, so the opening machinery has to be oversized, with higher running costs than a balanced design. Like the Gateshead Millennium Bridge, it would have been unique and expensive.

Of the other designs, both those from Price & Myers and Future Systems suffer from inaccessibility: steep approaches unsuitable for mobility-impaired users. They seem to be futuristic visions out of context to the site, creating technological challenges for their own sake rather than as a response to clear functional needs.

The Ian Ritchie design is a retractable bridge, an unusual opening variant often shunned because of the amount of space required on the river bank - although by overlapping that space with a permanent ramp, they have minimised the difficulty. Blue Office's inflatable truss would be the sort of thing to brighten the hearts of knife-wielding East-End vandals bent on puncturing architectural ambition.

The McDowell and Benedetti design is a swing bridge with an unfortunate lump and, I would guess, a problem inherent to the cable-stayed swing bridge whereby massive twists and inertias are generated during operation, as the cables provide no lateral stiffening to the supported deck.

As ever, links are only provided if a firm has project-specific information on their website. Click on the images for larger versions. I have only incomplete information on the competition, so if you are aware of any errors or omissions, send a comment.

I can't get my head round what went on with the Heatherwick design - the images available show an outlandish and impractical design with the deck supported on "ribbons" from adjacent buildings. But planning drawings dated March 2007[PDF] and with WSP and Heatherwick's names on them show a far more conventional cable-stayed bridge with no support from the buildings. It was reported in 2008 that Heatherwick's design had in turn been dropped in favour of a more conventional solution by the wider scheme architect SOM, so perhaps the drawing shows that design. SOM's own images of the scheme on their website continue to show the Heatherwick proposal ... confusing!

In any event, the bridge remains unbuilt and is likely to remain that way while the credit crunch continues.

20 February 2009

Regular readers will know that the Happy Pontist concentrates on bridges news in the UK, and only occasionally ranges further afield. With that in mind, it's nice to be able to cover a proposed new footbridge from nearly as far afield as is possible: New Plymouth, in New Zealand.

In May last year, it was announced that New Plymouth's Coastal Walkway would receive a 4km extension, including a new 68.7m footbridge over the Waiwhakaiho River. The bridge, pictured above right, is being designed by local firm Novare Design, who have kindly sent me several preview images showing the design as a work-in-progress (i.e. there are quite a few details which are likely to change).

The unusual steel arch design won a design-and-build lump sum competition, with the designer estimating the cost at less than US$4,000 per square metre of deck. The site is important in Māori history, being a place where the tangata whenua resisted occupation of their land by the Pākehā in the late 1870s.

The bridge's bleached skeletal design owes a clear debt to Santiago Calatrava, albeit more organic in feel and less high-tech. That impression is in great part due to the way the arch hangers extend above the arch itself, making it hard to see the pairing as anything other than ribs and spine, the remains of some long extinct whale perhaps.

The arch passes over the deck diagonally, although the deck is supported on one edge only, which gives rise to the somewhat peculiar rib geometry, whereby the hangers have to curve increasingly towards one end of the deck.

Visually, I like the asymmetry that this introduces, although structurally it's far from efficient. The edge support will lead to a substantial torsion in the deck which has to be restrained at each end, while the angle and curvature of the hanger ribs creates both a tendency towards horizontal sway (like a swing suspended above its lowest point) and the need for the hangers to resist substantial bending moments. For a roughly 70m span, I'd guess all these effects remain within a reasonably economic range to be worth accommodating in a landmark footbridge.

The bridge behaves as a tied arch, although the large armatures at the ends of the arch spine give more of the impression of a thrust arch - I find this a little odd, but visually, it may look better than the "wedding-cake" approach sometimes favoured by Calatrava where the structure is poised precariously on supports of a very different form (e.g. Puente del Campo Volantin).

Overall, it's an attractive spin on a classic bridge form, tweaked sufficiently to be interesting, but not so far as to require extraordinary measures to maintain stability.

Commenting on the way the bridge was procured, the designer, Peter Mulqueen, notes:

"With regard to lump sum competitions for foot bridges, I think there always needs to be a set of vibration criteria explicitly outlined in the competition documents. The competition should allow for this level of preliminary design in its compensation to entrants. Otherwise one ends up competing against super skinny structures that do not have a hope of remaining stable under pedestrian and wind loads, but are very material efficient and often cheaper."

In the UK, footbrige design competitions normally involve a basic requirement that designs comply with current standards (which cover vibration), and are stable against lateral excitation in particular (the latter requirement being a legacy of the Millennium Bridge's wobble). However, it's unclear in how many cases competitors actually carry out these checks - often it depends on the risk-reward ratio inherent in the competition setup.

For open competitions where large numbers of entries might be expected, only a tiny fraction would bother to do any structural analysis at all, let alone derive predicted deck frequencies or accelerations. For invited competitions, most competent firms would carry out dynamics calculations, although in some cases (where the simple checks show a possible problem), the proposal will be made to use tuned mass dampers to deal with vibration, rather than to amend the form of structure to eliminate the problem.

In theory, design-and-build competitions like the Waiwhakaiho one should be ideal, as the risk of having to remedy any vibration problem falls on the contractor, who is likely to be extremely risk averse and insist their designer address it at the outset. Regardless, if it's going to be a concern for the client, they should be seeking assurances about the design right from the start.

ICE historic bridge awardsThese were announced some time ago, but the newsletter of the ICE's Panel for Historic Engineering Works (PHEW) appeared more recently and included lots of detail on last year's winning schemes. The engineering ingenuity that goes into conserving historic structures is often far more challenging than that applied to new designs!

I've previously discussed David Billington's ideas on bridge design, which suggest that for a bridge to be "good", it must be elegant, efficient and economic. Like Leonhardt and Menn, Billington is opposed to extravagance, and to bridges which pay little heed to a structural optimum.

Virlogeux does not agree:

"Of course, David Billington is strongly influenced by the American philosophy of life, based on the individuals and on economy ... Economy has been too much the unique goal of narrow-minded engineers, resulting in some poor, ugly and repetitive structures which discredited the profession ... Engineers used to live in their narrow professional world, sure of a legitimacy based on rationality and competence. They have not been able to feel the evolution of our Society and the growing power of politicians and media, and of the lobbies which are able to influence them."

While advocating that engineers adopt a wider view, this is not to detract from "good" structural engineering, but to ensure that structural engineers have a greater voice in design:

"Structural engineers failed in having access to those who take the decisions, and it is not surprising that the result is often poor as regards structural projects in the recent years. The most striking is that design competitions have been organized for some bridge projects - when bridges are the essence of engineering art - which were only open to architects or in which engineers had only a limited role and no responsibility. Resulting in some clear disasters."

In essence, Virlogeux still adheres to the engineering party line, that structures should be efficient and never dishonest. Writing about Marc Mimram's Solférino Footbridge (pictured right, image courtesy of Etienne Cazin on flickr), he comments that:

"fantasy governed the detailed design, a fantasy which had not been tempered by the rationality of a serious engineer; the arches, for an example, are made of inclined I-shaped beams with totally irrational details. There is no web - for transparency - just a series of diaphragms, and the members are transversely curved, [which] obliged these beams [to be] made from cast steel elements ... back to the 19th Century and to enormous costs."

Virlogeux's prescriptions for a successful bridge design are that it be:

structurally efficient, and sometimes innovative

expressive of the state-of-the art in construction methods

built with perfection and elegance as a goal

built by the most eminent builders

in agreement with its surroundings

As may be obvious, Virlogeux is refreshing in his willingness to criticise his fellow engineers. From the same paper, there comes this example:

"We take an example from the World Exhibition in Sevilla to evidence such a difference, with the two major bridges built on this occasion: the Alamillo Bridge, which can be considered an attractive sculpture, but which is a total nonsense structurally, and the Barqueta Bridge [image courtesy of Guillermo Vale on flickr] designed by Juan Jose Arenas, a real structure which shows the natural flow of forces, and which is elegant and efficient in the same time."

So, it turns out, that while Virlogeux does not think engineers should be slaves to economy, there is nonetheless a moral imperative to avoid structural inefficiency:

"These monstrous errors must be systematically denounced to avoid their repetition, and to convince that excellence in bridge design can only come from a rational structural organization. Unfortunately, even some good architects who used to work efficiently with engineers are going in the wrong direction when they are given the responsibility of design. What is wrong? - too much searching for originality, aiming at producing a surprise more than trying to develop a pure structural design which is not, by essence, in their real competence.

"Modesty must be considered again a quality!"

This sense of anger seems to drive many bridge engineers commenting on bridge aesthetics in the last couple of decades. It is not entirely directed at the increasingly key role of architects in landmark bridge design, but also at clients and the wider society for promoting that focus. Hardly anyone wins a bridge design competition with a simple, efficient bridge of a form that has been used many times before. The anger is also provoked by a sense of loss, as bridge engineers are simply not used to having their controlling role in design usurped by others.

Ultimately, however, these debates are not about the interests of architects or engineers, but about whether bridge design serves the wider public well. The public do not care if a bridge design is structurally inefficient - but they do care if it is ugly, or if it requires far more of their money than an alternative. Virlogeux's focus on the principles of good structural design is only helpful where it serves these demands.

So, from all of this, is there any useful guidance as to how we can evaluate good bridge design, how we can criticise poor design? The expressed philosophies of the four designers I've highlighted are all subtly different, but all fundamentally derived from modernism and a belief that efficiency or economy is a moral rather than a purely commercial requirement. Personally, I think there should be room for idiosyncrasy, eccentricity, bravado, and even humour from time time - so long as there is accountability, and people understand both what they are paying for, and what the alternatives really are. This isn't the case as often as it should be.

Calatrava seems to have paid little heed to the Vitruvian ideals that he espoused twenty-five years ago, and his willingness to ignore prevailing wisdom remains admirable however ridiculous some of the resulting structures have been. There's room in this world both for Calatrava's flamboyant structure-as-sculpture, and for the elegant exploitation of structural behaviour that characterises many other bridge designers. I think the challenge is to make the public, and the clients who spend their money, really understand the choices that are available, and the real consequences of those choices in cost and risk. That will require better procurement processes, more visible structural engineers, and undoubtedly more public criticism of bridge design, both good and bad.

Some engineers are highly prescriptive in their views on bridge aesthetics, and Menn is undoubtedly one. In his paper "Functional Shaping of Piers and Pylons" (in Structural Engineering International, 1998), Menn wrote:

"A truly well designed bridge balances economy and aesthetics while responding to the functional requirements and technical and environmental boundary conditions."

"On the basis of the above considerations, the real art of bridge design is to elaborate a suitable technically appropriate structural system that aims at achieving an optimal balance of economy and appearance …This pragmatic, simple and purely functional approach not only leads to technically proper structures but also to aesthetically convincing ones."

Menn’s opinion is common amongst bridge engineers, with one typical example being the Billington & Woodruff paper discussed in a previous post. This moralistic position is also shared by Leonhardt.

Menn’s is the language of moral puritanism – bridges must be "proper"; ornamentation is improper; cost must be minimised; the most appropriate structural system will inevitably produce the best bridge. This back-to-basics approach continues to offer much of value in an age where architect-led bridge design has produced schemes which are unaffordable or unmaintainable, but it should not be the only game in town.

It's doubtful that in the modern era the assumption that a "simple and purely functional approach" automatically leads to "aesthetically convincing" structures. Writers such as Billington offer the greatest praise for the structures of designers such as Maillart, Candela, Nervi or Isler, but the conditions of production for such structures have changed irrevocably. These structures, for which the identification of an optimal form allowed forces and materials to be minimised, come from an age where least cost arose from least materials, and hence a technically efficient design would often coincide with one which is slender and elegant. Even in this pre-modern period, however, there were structures which are highly inefficient structurally yet which have become much loved icons, such as the Forth Railway Bridge (pictured above right, courtesy of Simon Bradshaw at flickr).

Modern technology has changed the conditions for least cost. In particular, mass-production, pre-fabrication, and automation mean that in most cases the least-cost solution is one that minimises site labour and maximises the use of off-site fabrication and assembly. In this situation, a parallel-flanged beam may cost less than a beam shaped to fit its bending moments, even though more material is used. Many structures which were efficient to construct have also been found to be expensive to maintain, and a structure with a lower whole life cost may well have higher initial cost e.g. the use of concrete and hence heavier foundations to avoid the cost of repainting structural steelwork.

The further difficulty with Menn’s prescription is the issue of public opinion. While there may be some common ground amongst structural engineers as to what constitutes a good design, it is far from clear whether our idea of good design is shared by the public who benefit from a bridge and who fund its construction. For example, the public may be thought to delight in Calatrava’s white skeletal frameworks, even though the costs of fabricating these complex geometries must frequently result in a project that greatly exceeds Menn’s "least expensive functional solution".

Who are engineers to judge the success of such a bridge if their opinion departs from the end-user? While clearly there are areas where engineers have expertise the public do not (on both capital cost and particularly on the likely maintenance liabilities), we should not deceive ourselves into thinking that a concentration on functionality automatically produces the "best" bridge.

14 February 2009

In 1995, the London Docklands Development Corporation held a competition to design a 200m long footbridge across the Royal Victoria Dock, connecting Silvertown on the south to the ExCel exhibition centre on the north. Central to the challenge was the need to maintain a substantial clearance to tall-masted ships, with the result that the bridge had to be at least 15m up in the air to allow for normal use. As a result, all the competition submissions are for a relatively unusual configuration of bridge.

The competition was won by Techniker and Lifschutz Davidson, with an innovative and undoubtedly spectacular 128m span "inverted Fink truss" bridge. Their design featured a 40-person transporter gondola at low level, so that pedestrians could cross the waterway without first having to use a lift or the stairs. Sadly for fans of transporter bridges, this element has never been installed, as foot traffic just doesn't justify it.

I greatly admire the attempt to rehabilitate an archaic railway bridge design (technically it's not really an inverted Fink truss as several diagonal ties are omitted - it's almost more of a variant cable-stayed bridge), and in elevation (see images below), the superstructure design is a very striking form. The supports are far less successful, especially the very monolithic enclosed stair and lift towers, and overall the scale is perhaps too much, it just towers too much above the surroundings.

The deck's extreme slenderness is also only maintained at the cost of somewhat unsightly bulges above walkway level, apparent on the photograph (above right), which also shows the use of external cables near the parapets for, err, unauthorised maintenance access.

Of the other competition entries, I find the Robert Benaim one somewhat baffling as it is not at height and is unclear how ships are allowed for - perhaps it's from an unrelated competition! The curved tie to which most of the stays are attached looks pretty but structurally all it adds is a huge problem if cables ever need to be replaced.

The Cezary Bednarski design seems the most successful at resolving the difficult end towers, something the Price & Myers proposal does very poorly. I like the Richard Horden submission, especially how it begs the question of whether an opening span would be a much better approach for a waterway which I imagine only rarely accommodates high-masted vessels.

I've tried to find images of the unsuccessful entries on the internet, but so far only found four. I don't know if there were more. If anyone can point out others to me, I'll update this post in the future.

As ever, links are only given if there's more information on a design on a firm's website. Click the images for a full-size version.